1. Field of the Invention
The present invention relates to a target substance detection kit and target substance detection method.
2. Description of the Related Art
Recently, methods for easily detecting a trace quantity of magnetic particles using a magnetoresistive device have been proposed, the magnetic particles being used as marker objects (see, for example, David R. Baselt, et al., Biosensors & Bioelectronics 13, 731, 1998; D. L. Graham, et. al., Biosensors & Bioelectronics 18, 483, 2003).
David R. Baselt, et al. (1998) used giant magnetoresistive (GMR) devices measuring 80 μm×5 μm and 20 μm×5 μm as a magnetic sensor to detect multiple magnetic particles 2.8 μm in diameter. Magnetic films used for GMR devices are in-plane magnetization films and a magnetic field applied to the magnetic particles is applied to the magnetic films in a direction perpendicular to film surfaces. Consequently, magnetic stray fields emitted by the magnetic particles magnetized by the application of the magnetic field are applied to the magnetic films of the GMR devices approximately along a film plane and magnetization of the magnetic films is aligned with a direction of the magnetic field. The magnetic field used to magnetize magnetic particles in the manner described above is generally known as a biasing magnetic field.
Magnitude of electrical resistance of a magnetoresistive device depends on relative magnetization directions of two magnetic films. Specifically, the electrical resistance is relatively small when the magnetization directions are parallel and is relatively large when the magnetization directions are antiparallel. To bring about parallel and antiparallel magnetization states, one of the two magnetic films of the magnetoresistive device has its magnetization direction fixed and the other magnetic film is made of a magnetic material which has such a coercive force that the magnetic field of the magnetic material can be reversed by the magnetic stray fields of the magnetic particles. If no magnetic particle exists on the magnetoresistive device configured in this way, no magnetic reversal occurs because application of a biasing magnetic field will not cause a magnetic field to be applied along the film plane.
Also, a detection circuit proposed by David R. Baselt, et al. (1998) includes a bridge circuit made up of two fixed resistors, a GMR device on which magnetic particles are not fixed, and a GMR device on which magnetic particles can be fixed. The detection circuit detects a potential induced in the bridge circuit, using a lock-in amplifier.
D. L. Graham, et. al. (2003) used GMR devices measuring 2 μm×6 μm with in-plane magnetization films to detect a magnetic particle 2 μm in diameter. As is the case with David R. Baselt, et al. (1998), D. L. Graham, et. al. (2003) detected a magnetic particle by comparing output signals of two GMR devices placed side by side: a GMR device on which a magnetic particle can be fixed and a GMR device on which a magnetic particle is not fixed. However, a magnetic field was applied to the magnetic particle in a longitudinal in-plane direction of the magnetic films.
As described above, methods for detecting magnetic particles using magnetoresistive devices detect the magnetic particles by magnetizing the magnetic particles in a desired direction and varying the magnetization direction of the magnetoresistive device using magnetic stray fields emitted by the magnetic particles. These methods are easy to handle and enable detection in a relatively short time.
The magnetic particle is fixed to a sensor, for example, using an antigen-antibody reaction if a target substance to be detected is an antigen. Specifically, a primary antibody formed on the sensor is allowed to react with a specimen such as blood which may contain an antigen. Then the magnetic particle modified by a secondary antibody is allowed to react with the specimen. If there is an antigen in the specimen which is obtained by the series of operations, there will be binding among the primary antibody, antigen, secondary antibody, and magnetic particle. If there is no antigen, no such binding will occur, and consequently, the magnetic particle will not be fixed to the sensor.
With this fixing method, one magnetic particle per target substance is fixed to the sensor, and thus a single target substance can be detected by means of a highly sensitive magnetic sensor.
Being similar to the sensing method using a magnetoresistive device, a method for detecting magnetic particles using a Hall device as a magnetic sensor has been proposed (see, for example, Pierre-A. Besse, et al., Appl. Phys. Let. 22, 4199, 2002).
Pierre-A. Besse, et al. (2002) magnetized a 2.8-μm-diameter magnetic bead placed just above a Hall device by the application of a DC magnetic field, changed the magnetization direction of the magnetic bead by the application of an AC magnetic field, and thereby detected the magnetic bead. When a magnetic field is applied in a z-axis direction with an electric current being passed along the film plane direction of the Hall device, electrons are subjected to a Lorentz force, causing a potential to be produced in a direction orthogonal to the electric current in the film plane.
The potential is proportional to magnetic field strength, and thus the potential due to Hall effect changes with changes in the magnetization direction of the magnetic bead. Since no magnetic stray field is produced without a magnetic bead, the magnitude of the magnetic field applied to the Hall device varies with the presence or absence of a magnetic bead, and so does the magnitude of the potential. This means that the presence or absence of a magnetic bead can be detected by a Hall device.
Also, a method which detects magnetic particles using a superconducting quantum interference device has been proposed (see, for example, K. Enpuku, “Biological immunoassay with magnetic marker and SQUID magnetometer,” OYO BUTURI Vol. 73, No. 1 (2004), p. 28 (the Japan Society of Applied Physics)). According to this method, the magnetic particles are fastened to a detection area and aligned in terms of their magnetization direction by the application of a magnetic field. The magnetic stray fields emitted by the magnetic particles are detected by a Josephson device to determine quantity of the magnetic particles. However, to eliminate impacts of the applied magnetic field, the superconducting quantum interference device has to be placed parallel to the applied magnetic field so as not to cross the applied magnetic field.
To detect magnetic particles, it is desirable that the magnetic stray fields emitted by the magnetic particles are large, but small magnetic particles, whose magnetization is not saturated in zero magnetic field, produce small magnetic stray fields. Also, in the medical field, when desired biomolecules are extracted from a specimen solution such as blood or when magnetic particles are used for a biosensor currently under study, desirably the magnetic particles are dispersed when dropped in the specimen solution. For that, it is desirable that the magnetization is very weak. In zero magnetic field, some magnetic particles are superparamagnetic without magnetization. To detect such weakly magnetized magnetic particles, it is necessary to align the magnetization of the magnetic particles in one direction by the application of an external magnetic field as described above.
However, it is not desirable to detect minute magnetic stray fields emitted by magnetic particles by applying a large magnetic field to the magnetic sensor. To detect particularly small magnetic stray fields, it is necessary to devise measures such as preparing two magnetic sensors, fixing magnetic particles to only one of the magnetic sensors with the other magnetic sensor used as a reference, and detecting a difference in signals of the two sensors by means of a sense amplifier. Furthermore, when using magnetic sensors which give a small detection signal, it may also be necessary to incorporate the magnetic sensors into a Wheatstone bridge circuit. When using a magnetoresistive device or Hall device as a magnetic sensor, it is necessary to strictly control the direction of a biasing magnetic field to reduce variation in the detection signal. This is true especially when using a superconducting quantum interference device which has very high magnetic field sensitivity.